Helical supramolecular organizations from metal-organic liquid crystals

The present paper is aimed at providing a survey of investigations undertaken regarding the preparation and characterization of helical supramolecular architectures using a strategy based on two concepts: coordination chemistry and mesomorphic behavior. Metal-coordination procedures have been used to design and synthesize molecular units capable of organizing into helical superstructures. In order to ensure this capability, the molecular building blocks have been complemented with a pro-mesogenic character, most commonly columnar, so that this type of mesophase is used as a template to build up helical supramolecular architectures with long-range orientational order and long-range positional order. For example, metal-containing liquid crystals have been described that organize into helical columnar mesophases whose structure has been fully investigated. To date, molecular units that display this organizational behavior have been found to be very different in shape, and hence the formation of helical structures has a different origin in each case. In this paper, three molecular structures are discussed (i.e. ‘disk-like’ molecules, ‘propeller-like’ molecules and polycatenar elongated molecules) that have been shown to give rise to helical organizations. Models have been proposed for the three systems described and in the present overview these have been classified as azimuthal rotation of planar disk-like complexes, interdigitated stacking of octahedral complexes and staggered stacking of polycatenar elongated planar complexes, respectively. Throughout this paper, emphasis is placed on studies carried out in the mesophase that have allowed authors to propose the corresponding models.